As an electrical power cable ages, the dielectric strength of its insulation is reduced, thereby shortening its life expectancy. As is well known in the art, a primary mechanism for the reduction of insulation strength with cable age is a phenomenon commonly referred to as water-treeing. Over time, water, on a molecular scale and in the presence of high electrical stresses and ionic contamination, can degrade the cable insulation's structure. This degradation typically occurs at discreet points along the cable length where point defects have increased the localized electrical stress level in the insulation. The degradation further increases discreet point electrical stresses and decreases insulation strength leading to a progressive condition. As the condition grows out from the initial defect site, it often takes on a bush-like appearance giving rise to its common name: a water-tree.
Water-trees are slow growing structures which lead to cable failure, but they are not themselves the mechanism of cable failure. Failure occurs when a voltage perturbation of some sort, i.e., a lightning surge, a switching surge, etc., causes a partial breakdown in the cable's insulation at the sight of reduced strength and increased stress. That partial breakdown forms a new structure commonly referred to as an electrical-tree. The electrical-tree defect propagates through the insulation very quickly, often failing a cable within hours or days of initial inception. Thus, in summary, water-trees develop slowly, degrading the cables insulation until that degradation gives rise to an electrical-tree, which quickly propagates and fails the cable. Water-tree growth can occur on a timescale of decades but electrical-tree growth often leads to failure in a time frame of hours or days.
Water-tree growth can be stopped through any process which removes the ionic laden water from the site. The deleterious effects of the remaining water-tree structure can be reduced if the electrical stresses associated with the discontinuity of the water-tree structure are stress graded. There are chemical solutions (restorative or injection fluids) which, when injected into the conductor strands of a cable, diffuse into the cable's insulation and chemically combine with the water existing in the insulation's structural defect, thereby removing the fuel for further water tree development. In some restorative fluid processes the by-product of this reaction may serve to stress grade the point defects of the structural defect, thereby reducing its deleterious effects on the cable's insulation. In the most refined restorative fluid solutions, those by-products can be chosen such that they remain in place within the water tree structure forming a pseudo-permanent solution to the water-tree degradation.
Restorative fluid solutions intended only to mitigate the deleterious effects of water-tree growth have little effect on cables that contain electrical-tree structures. However, if a cable has been functioning properly and it has not been subjected to any service disruptions or any conditions that vary from its standard operating condition, it is very unlikely that the cable will contain an electrical-tree structure at the time it is chosen for restorative fluid injection. Because an electrical-tree typically fails a cable in a matter of hours or days, it is statistically very unlikely that any particular cable with a lifespan of 15 or more years of life will be, coincidentally, in its last days of life at the time it is selected for injection. A cable injection, when performed on a cable that has been functioning properly at standard operating conditions, is typically referred to as a pro-active cable injection.
However, if the aged cable has recently been subjected to any conditions outside of its standard operating conditions, the statistical safety net described above no longer applies. Moreover, because of the aged condition of cables being considered for chemical injection, it is impossible to estimate the specific factor of safety between the operating conditions under which the cable is currently functioning and the actual, unknown value of the ultimate dielectric strength of the cable's insulation. Without knowing the exact extent of the cable insulation's degradation it is not possible to know what level of perturbation above its standard operating condition it is able to withstand. Examples of such perturbations are:                surges due to switching the cable out and into service        application of voltages in excess of system voltage for analytical purposes        surges that occur during cable failures and fault location        application of DC voltage testing, resulting in space charge development        
A cable injection, when performed on a cable which has recently been subjected to any non-standard conditions or perturbations outside of its standard operating condition, is typically referred to as a re-active cable injection. Often the ‘non-standard condition’ is an actual cable fault. In that case the decision to inject the cable is a reaction to the obvious weakness of the cable indicated by the fact that it has already faulted at least once. However, any one of a number of perturbations or deviations from standard operating conditions, such as those listed above, would classify the injection as a re-active cable injection.
As one would expect, the failure rate for cables injected in a re-active manner is significantly higher than the failure rate for cables injected in a pro-active manner.
Injecting a cable that already contains an electrical tree is undesirable since ultimately the materials and labor are wasted on the effort. Detecting the presence of an electrical tree before injection is; therefore, highly desirable. Fortunately, between the inception of an electrical tree structure and the time that the electrical tree actually fails the cable, its presence can be detected using any test methodology that can detect the presence of an active partial discharge within the cable insulation. Additionally, tests which are sensitive enough to detect the partial discharging of an active electrical tree are also typically able to detect other electrical system discharge activities such as arcing, surface discharging, etc. These discharges also indicate issues that one would want to be aware of and perhaps address before injecting a cable.
The cable rejuvenation injection protocol of the present disclosure incorporates analytical methods of detecting active partial or full discharge activity; hereinafter sometimes collectively referred to as an “electrical discharge” or “discharge” occurring at system voltage into a cable rejuvenation injection process. Such methods include specialized test procedures aimed at eliminating the possibility of injecting a cable that already contains an electrical tree discharging at or about system voltage. The cable rejuvenation injection protocol may also include a step for checking the cable immediately after it has been chemically injected and returned to service in order to assure that any craftwork or switching process performed in the standard course of the cable rejuvenation injection process did not incite a new electrical-tree or other cable or system defect.
The testing performed as a part of this overall cable rejuvenation injection protocol should be distinguished from test protocols that are designed to evaluate the general condition of a cable for the purpose of prioritizing the replacement sequence of a population of cables. Currently there are a number of testing devices and procedures available that are used to try to ascertain a qualitative analysis of a cable insulation's health. Often, the goal is to prioritize cables for replacement. The cables that have the most degraded insulation are prioritized for replacement while the cables with the least aging are allowed to remain in service.
A typical test isolates the cable from the rest of the system and then uses an exterior power supply to incrementally raise the voltage of the cable in multiples of its operating voltage. Many of these tests use some indicator to determine at what voltage an electrical discharge occurs. In the specific case of water tree evaluation, different technologies take advantage of different phenomenon to make this determination. But most tests that work on this principle ultimately incite a discharge event by increasing the test voltage and then drawing conclusions about the cable's condition based largely on the size of the gap between the operating voltage of the cable and the inception voltage of the discharge incited by the test.
Unfortunately, these tests are not helpful at determining cable injection suitability. These tests typically require that the cable be switched out of service for the testing to be performed. Additionally, they require that overvoltage be applied to the cable to incite a discharge event. Therefore, the tests themselves create circumstances that take a cable from its proactive state to a reactive state. Moreover, the objective of these tests is to identify and replace the cables in the most aged, water treed condition. These are the cables that are the most likely to be seriously damaged by this testing approach. However, since the test is being used to determine the priority for cable replacement, the potential damage to the cable caused by the test can be considered a reasonable risk. In comparison, an objective of the injection protocol of the present disclosure is to rejuvenate the most aged (highly water-treed) cables, which are the cables most likely to be damaged by the overvoltage testing and rendered un-suitable for injection. Thus, it can be appreciated from the foregoing that there is a need for an improved method for determining cable injection suitability performed at the time, and as a part of the cable rejuvenation injection process.